1. Field of the Invention
The present invention relates to a method of transmitting digital signals between digital video and audio signal recording and reproducing apparatuses by making use of bit rate reduction technology.
2. Description of the Prior Art
Recent dramatic advancements in the video and audio signal recording and reproducing technology make use of digital techniques, but when attempts are made to apply such technology to the home-use VTR, the most significant problem that arises is with respect to the data quantity. For example, the transmission rate of the video signal to be delivered is as high as 216 Mbps in the 4:2:2 standard, which renders direct long-time recording difficult.
Accordingly, various bit rate reduction methods have been proposed for the video signals and audio signals, and recording and reproducing apparatuses making use of bit rate reduction techniques have come to be developed.
As for bit rate reduction, for example, the video signal is realized by the discrete cosine transform proposed by C. Yamamitsu et al. in "An Experimental Study for a Home-Use Digital VTR," IEEE Trans. on Cons. Elec., Vol. 35, No. 3, pp. 450-457, Aug. 1989, and the audio signal is represented by the sub-band coding method as disclosed in "Low Bit-rate Digital Audio Coding Systems," CCIR Report 1199, 1990.
Meanwhile, as a problem associated with bit rate reduction, the issue of accumulation of distortions by recoding is known. For example, in video signals, generally the frequency components obtained by orthogonal transform of the intended signal are coded, but in the orthogonal transform and inverse orthogonal transform, rounding errors may occur in relation of the number of operation digits and number of output digits. In audio signals, a similar operation error may be anticipated.
In the conventional digital audio appliances or digital video equipment, A/D-converted audio signals or video signals are transmitted. The conventional digital signal transmitting method is explained below.
Digital signal transmission is performed between recording and reproducing apparatuses comprising a video signal A/D converter, a bit rate reduction encoder, an error correction encoder, an audio signal A/D converter, an audio signal encoding processor, a modulator, a magnetic recording and reproducing system, a demodulator, an error correction decoder, a video signal concealment processor, an audio signal decoding processor, an audio signal concealment processor, a bit rate reduction decoder, a video signal D/A converter, and an audio signal D/A converter.
The input video signal is A/D-converted by the video signal A/D converter, and is encoded by the bit rate reduction encoder, and parity symbols of error correcting codes are added in the error correction encoder. On the other hand, the input audio signal is A/D-converted by the audio signal A/D converter, and is sent into the audio signal coding processor for audio signal processing such as shuffling and error correction encoding, and is multiplexed on the video signal. The signal is then modulated in the modulator, and is recorded in the magnetic recording and reproducing system.
The data reproduced from the magnetic recording and reproducing system is demodulated in the demodulator. Errors which occurred in the magnetic recording and reproducing system are corrected in the error correction decoder, and uncorrectable errors are concealed in the video signal concealment processor. Then, bit rate reduction decoding is effected in the bit rate reduction decoder. The signal obtained at this point is used as the video signal portion of the transmission output. After demodulation, audio signal processing such as deshuffling and error correction decoding is effected in the audio signal decoding processor, and uncorrectable errors are concealed in the audio signal concealment processor. The signal obtained at this point is used as the audio signal portion of the transmission output.
In the final output, the video signal D/A-converted by the video signal D/A converter and the audio signal D/A-converted by the audio signal D/A converter are delivered.
To transmit from the reproducing side apparatus to the recording side apparatus, the outputs of the bit rate reduction decoder and the audio signal concealment processor of the reproducing side apparatus are transmitted to the recording side apparatus, and they are respectively fed into the bit rate reduction encoder and audio signal encoding processor.
In such a constitution, however, for one transmission, the number of times of orthogonal transform and inverse orthogonal transform on the original signal increases by one each, and the distortions due to orthogonal transform and inverse orthogonal transform are accumulated, and accordingly distortion due to bit rate reduction may also take place.
The transmission rate of the output video signal is, for example in the 4:2:2 standard, 216 Mbps, which means serial transmission is difficult. In parallel transmission, however, the problem arises with respect to the shape of the cable used in transmission. Still worse, since the transmission of the audio signal is not included in the standard of the video signal, more serious problems with respect to cable and transmission rate are presented.